1. Field of the Invention
This invention relates generally to marine seismic surveying, and, more particularly, to source and receiver side wave field separation for seismic data acquired in a marine seismic survey.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, such as the exemplary survey 100 conceptually illustrated in FIG. 1, marine seismic streamer 105 is towed behind a survey vessel 110. The seismic streamer 105 may be several thousand meters long and contain a large number of sensors 115, such as hydrophones and associated electronic equipment, which are distributed along the length of the each seismic streamer cable 105. The survey vessel 110 also includes one or more seismic sources 120, such as airguns and the like.
As the streamer 105 is towed behind the survey vessel 110, acoustic signals 125, commonly referred to as “shots,” produced by the seismic source 120 are directed down through the water column 130 into strata 135, 140 beneath a seafloor 145, where they are reflected from the various subterranean geological formations 150. Reflected signals 155 are received by the sensors 115 in the seismic streamer cable 105, digitized, and then transmitted to the survey vessel 110. The digitized signals are referred to as “traces” and are recorded and at least partially processed by a signal processing unit 160 deployed on the survey vessel 110. The ultimate aim of this process is to build up a representation of the subterranean geological formations 150. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations 150.
Seismic bandwidth may be limited by source and receiver ghosts caused by reflection of the acoustic signals 125 at the surface 165. Seismic bandwidth is the width of the amplitude spectrum, which may be defined as the frequency difference between the highest and lowest frequency at which the amplitude is above 6 dB. When the seismic bandwidth is larger, more frequencies contribute to the signal, which may increase the temporal resolution. On the source side, the effective source signature is a combination of the source signature and at least one source ghost formed when the source signature is reflected by the surface 165 before traveling to the sea floor 145. On the receiver side, the recorded pressure wave field is a combination of the up-going wave field traveling from the seafloor 145, which includes the source ghost, and a down-going wave field traveling from the surface 165 that includes the receiver ghost.
Receiver ghosts in the seismic data recorded by the receivers 115 may be at least partially removed, or de-ghosted, using a marine seismic survey that includes data collected by the streamer 105 when deployed at both a shallow depth and at a greater depth. For example, the streamer 105 may be deployed at a depth of 6 meters during a first pass over the seafloor 145 and then may be deployed at a depth of 9 meters during a second pass. Alternatively, the receivers 115 may be deployed at varying depths. This arrangement of the streamers 105 is sometimes referred to as an over/under combination of the streamers 105. The term “over” is typically associated with the shallow streamer 105 and the term “under” is typically associated with the deep streamer 105. The over/under combination technique is also known as a dual-streamer de-ghosting technique, an acoustic wave field decomposition, and the like. Moreover, the vertically-separated seismic sensors 115 may be referred to as a vertical receiver array.
FIG. 2 conceptually illustrates an alternative embodiment of a conventional system 200 that may be used to perform a marine seismic survey using an over/under combination technique. The system 200 includes a survey vessel 205, which tows a shallow streamer 210(1) and a deep streamer 210(2). The shallow and deep streamers 210(1-2) each include at least one receiver 220(1-2). A source 215 provides a seismic signal 225 that is received by receivers 220(1-2). As indicated in FIG. 2, the source 215 is typically deployed at a different depth than the receivers 220(1-2). One or more receiver ghost signals 230(1-2) are also received by the receivers 220(1-2). Thus, seismic data acquired by the receivers 220(1-2) includes contributions from at least the seismic signal 225(1-2) and the one or more receiver ghost signals 230(1-2).
FIGS. 3A-D conceptually illustrate portions of received seismic signals. In particular, FIGS. 3A and 3B conceptually illustrate a seismic signal that may be received by the shallow streamer 210(1) as a function of time (in FIG. 3A) and as a function of frequency (in FIG. 3B). As shown in FIG. 3A, the seismic signal includes an up-going wave field 310, which is approximately a delta-function corresponding to a flat amplitude spectrum seismic signal 315 in the frequency domain shown in FIG. 3B. A down-going wave field 320, corresponding to a receiver ghost signal, is depicted in FIG. 3A as an approximate delta function with a negative amplitude that arrives at a later time than the up-going wave field 310. The “over” recorded seismic data 325 acquired by the shallow streamer 210(1) is a combination of the up-going wave field 310 and the down-going wave field 320. Accordingly, the “over” recorded seismic data 325 may include one or more notches 330 that may not be present in the flat amplitude spectrum seismic signal 315.
FIGS. 3C and 3D conceptually illustrate a seismic signal that may be received by the deep streamer 210(2) as a function of time (in FIG. 3C) and as a function of frequency (in FIG. 3D). As shown in FIG. 3C, the seismic signal includes an up-going wave field 350, which is approximately a delta-function corresponding to a flat amplitude spectrum seismic signal 355 in the frequency domain shown in FIG. 3D, and a down-going wave field 360, corresponding to a receiver ghost signal, which is depicted in FIG. 3C as an approximate delta function with a negative amplitude that arrives at a later time than the up-going wave field 350. The “under” recorded seismic data 365 acquired by the source 215(2) on the deep streamer 210(2) includes one or more notches 370 that may not be present in the flat amplitude spectrum seismic signal 355.
The notches 330, 370 may result in resolution loss in the acquired seismic data. Thus, over/under combination technique attempts to estimate the up-going and down-going wave fields 310, 350 and 320, 360 by combining the “over” recorded data 325 and the “under” recorded data 365. For example, the up-going wave field 350 and a down-going wave field 360 of the deep streamer 210(2) are separated by a different time lag than the up-going wave field 310 and the down-going wave field 320 of the shallow streamer 210(1). The location of the notches 330, 370 depends on the depth of the streamers 210(1-2) and, consequently, the frequencies of the notches 370 are different than the frequencies of the notches 330. This property may be used to combine the “over” and “under” recorded data 325, 365 to reduce the effect of the notches 330, 370 in the combined data set. However, these conventional methods do not account for source ghosts.
Over-under source configurations have been used to attempt to attenuate source ghosts in ocean bottom cable data. However, these techniques rely on an approximate solution based on vertical seismic profiling (VSP) processing. Moreover, receiver side de-ghosting cannot be performed with ocean bottom cables, since these cables typically rest on the sea floor and thus cannot be deployed in an over-under configuration. Consequently, conventional methods of de-ghosting seismic data are not able account for both receiver and source ghosts, as well as the interactions between them, such as the effect of source ghosts on receiver-side seismic data.
The present invention is directed to addressing the effects of one or more of the problems set forth above.